American patent No. 4,850,722 (FAG) and French patent No. 2,629,155 (SKF GmbH) disclose sealing devices for a bearing into which are built encoding elements intended to generate flux variations in a magnetic field in the vicinity of a passive sensor, creating, in the latter, signals in the form of a variable current which signals make it possible to determine the characteristics of rotation of the rotating part of the bearing, such as speed of rotation, angular acceleration or angular displacement, with respect to the non-rotating part of the bearing.
The major drawback of these conventional devices is that they operate in interaction with a passive sensor of the inductive type generally including a magnet, a polar core and a coil and the use of which involves a relatively large size, a high sensitivity to the variations in air gap between the sensor and the target creating the flux variations in the magnetic field. In addition, with this sort of device, the amplitude of the signal supplied by the sensor is too low to be used at low speeds of rotation close to zero, which corresponds, for an application to a wheel bearing equipped with an anti-lock braking system of the ABS type, to a vehicle speed of less than approximately 5 km/h.
A known solution for overcoming the aforementioned drawbacks consists in using, as a type of sensor, Hall-effect probes or magnetoresistive sensors. The encoding element interacting with this type of sensor must be capable of generating, in rotation, magnetic field strength variations. To do this, use is made of a multipolar ring secured to the rotating part of the bearing and passing rotationally in front of the sensor.
However, the use of a multipolar ring as encoding element has certain drawbacks in terms of the production of the said ring and of securing it to the rotating part of the bearing. These are, for example, problems of accuracy in manufacturing the multipolar ring, in magnetizing the segments of the multipolar ring, of temperature-related behaviour and differential thermal expansion with respect to the support of the ring.
Furthermore, European patent application EP No. 0,320,322 (SNR) discloses a bearing with a magnetic field detector in which the variations in magnetic field strength may be generated by the combination of a permanent magnet secured to the non-rotating part of the bearing and by an annular element with radial fins which are permeable to the magnetic flux and secured to the rotating part of the bearing. A magnetic field sensor is mounted on the non-rotating part of the bearing axially facing the permanent magnet, the element with fins passing rotationally between the permanent magnet and the sensor.
The detection device according to document EP 0,320,322 has the advantage of providing a signal which is stable because the distance separating the permanent magnet and the sensor is constant without a variation in air gap. However, this detection device is designed for a very specific structure of bearing in which the rotating part consists of a rotary shaft, without any preoccupation as regards sealing the bearing with respect to the external surroundings. The art disclosed in this document furthermore involves a high number of elements to be handled and to be positioned between the rotary shaft and the fixed support which surrounds it. That involves difficulties in assembly, in the accurate positioning of the elements, and in the electrical connection of the sensor (which electrical connection is not discussed in this document).